"Dale was a disciplinarian," Mangano said. "The next commissioner will be a disciplinarian, too."

Dale stepped down suddenly Thursday after Kathleen Rice, Nassau's district attorney, faxed Mangano's office a four-page letter detailing results of an investigation into outside political influence in the police department.

The letter said Dale, "in a judgment fraught with peril," had inserted himself directly into a politically charged civil case by ordering that a witness, Randy White, be arrested.

White, of Roosevelt, had offered testimony in a civil case potentially damaging to former Freeport Mayor Andrew Hardwick's bid to run a third-party campaign in this year's county executive race. Democrats said Hardwick's candidacy was intended to help Mangano's re-election campaign by draining black voters from Mangano's opponent, Thomas Suozzi.

The Hardwick campaign's sole financial supporter, Gary Melius of Huntington, called Dale a few days after White's testimony to say that the campaign wanted to file a perjury charge against White. Hardwick's supporters said they had an audiotape showing that White had lied. But police found the tape to be inaudible and declined to charge White with perjury.

Dale later ordered that White be arrested on an outstanding bench warrant for failing to pay a fine after pleading guilty to selling bootleg DVDs.

Mangano said he read the letter on Thursday and then called the police commissioner -- who has yet to comment on the resignation -- over to his office.

Mangano said he handed Rice's letter to Dale, who immediately tendered his resignation after reading it. "He chose to resign right away," Mangano said. "He's a Marine and he handled it like a gentleman."

Mangano said he would have demanded the resignation had Dale not offered it on his own.

"The letter brings into question some of the operation and procedures in the police department," Mangano said. "I felt like I had to bring in new leadership to make clear to the public that those questions would be addressed."

Mangano praised Dale's nearly two-year tenure, saying that the former New York City police administrator had made the department better by beginning to bring order to a department that had been stung by a series of scandals.

Early on, Dale said he believed Nassau's police unions had too much influence on how the department handled disciplinary actions. And he, with the support of Mangano and lawmakers, fought for the right to fire officers.

"Dale worked under a lot of pressure, he took a lot on and he took on a lot of people," Mangano said. "He may be gone, but the effort to instill discipline will continue."

Victor Politi, Mangano's deputy county executive for public safety, will serve only as interim commissioner, Mangano said.

Politi has been tasked with reviewing department procedures, from operations to the chain of command, in light of the district attorney's investigation, Mangano said.